SCP2 is a sex factor from Streptomyces coelicolor and SCP2* is a mutant with enhanced pock formation which associates with sex factor activity in streptomycetes. These low copy number plasmids can be transferred to a variety of streptomycetes by genetic crosses and protoplast transformations. Much of the plasmid DNA is not essential for replication or use of the replicon in vectors for recombinant DNA. The present invention extends the knowledge about SCP2* to derive several small, self-transmissible plasmids, identify a fragment containing the basic replicon which contains the necessary information for normal replication, identify DNA segments controlling the level of plasmid DNA, and identify the minimal replicon which is the smallest self-replicating segment of the plasmid.